....after all the driving force behind the Decepticons are dead and whatever is left is not a threat. There is no way to revive the dead 'Cons as The Matrix is with Optimus and he is unlikely to lend it out. It was stated that there is no other way to revive a Transformer. There is no room for a prequel as TF1 really doesn't allow for much to be told before hand unless it is on Cybertron - which is very unlikely. There is only slightly plausible way to continue and that is a new factionless threat that is too strong for the Autobots and requires Megatron to be revived and recruited for the greater good etc. Even this is unlikely given that the Military have shown themselves to be capable of killing Transformers with relative ease - tomahawk missiles, a parachute and a girl effectively unwound decades of Decepticon planning. So I think we have to accept that DOTM for better or for worse is the last TF movie we'll see for sometime.